The maximum size of a computer system's addressable memory is dependent on the size of the computer system's memory address bus. For example, if a computer system's memory address bus is 32 bits wide, the maximum size of the computer system's memory is 232 (4 Gigabytes) of memory. If a computer system's memory bus is 64 bits wide the maximum size of the computer system's memory is 264 bytes.
All of the computer system's addressable memory may not be physically present in the computer system. Memory physically in the computer system is called physical memory. The physical memory may be volatile memory such as Random Access Memory (“RAM”) or non-volatile memory such as Erasable Programmable Read Only Memory (“EPROM”).
A typical computer system includes memory management hardware for managing the addressable memory. The computer system's operating system, a sequence of program instructions resident in the computer system's memory, includes program instructions for controlling the memory management hardware. The operating system in conjunction with the memory management hardware manages the physical memory for applications executing in the computer system.
An application includes processes to be executed by a processor in the computer system. A process is a sequence of program instructions in the application. Other program instructions in the application start and terminate the process. Once the process starts executing it may request allocation of addressable memory from the operating system in order to perform the operations encoded in the program instructions.
Management of the computer system's addressable memory by the operating system includes allocating addressable memory to processes executing in the computer system. The allocation of addressable memory includes mapping addressable memory addresses to addresses of physical memory present in the computer system. The maximum addressable memory to be allocated by the operating system to a process is determined by the operating system. For example, a 32 bit operating system with a maximum addressable memory of 4 Gigabytes, such as Microsoft Windows NT operating system, executing in a computer system with a maximum addressable size of 264 bytes, such as computer systems utilizing Compaq Computer Corporations's Alpha Processor limits the per process maximum addressable memory to 2 Gigabytes.
A limit on the maximum memory allocated to a process imposed by an operating system impacts the performance of the application.